A Proven Hull Shape
by Don Andrews & Tim O'Leary
Perhaps with the availability of the new building plans, drawn by Jonty Sherwill and approved at OKDIA AGM in February 1986, we can expect a revival of interest in home building. Technical Manual 4, Building the OK Hull, contains a cutting list, cutting instructions and step by step building directions. It also includes some ideas on appropriate and acceptable modifications to the standard design, an alternative building technique to the traditional method, and a few fairly 'way out' notions to amuse the adventurous. Neither the manual nor the new plans aim to tell you what shape to build your boat. The home builder, it is assumed, will either build 'down the middle' or have his own personal formula for success, and indeed one reason for going to the trouble of home building is to test that formula.
Designing a 'fast' OK hull within its permitted tolerances has become something of a black art, mastered only by the privileged few, and quite a mystery to ordinary mortals. Some take the view that certain hull shapes possess almost magical powers. As measurers at international championships both the authors have been asked to scrutinise known 'fast' hulls particularly carefully so as to make it quite clear to all competitors that they were not being challenged by a Flying Dutchman in disguise.
One can, of course, take the view that hull shape is only the third most important contribution to boat speed, ranking below the competence of the helm and the efficiency of the rig. Hull shape obviously matters, even if you do rank it third in importance, but probably only within certain tolerances, and sometimes the helm's mastery of the conditions is such that he or she could win driving an OK hull of any shape that measured and was around minimum weight.
Some readily available GRP hulls, though, have a reputation for being 'proven' shapes, and as measurers the authors do have the opportunity to identify common denominators between these hulls. It is the purpose of this article to consider these common denominators, and to suggest to those home builders who don't have a personal formula to test ways of designing a competitive OK hull without putting themselves through some sort of magical initiation ceremony first.
Our findings are based on our measurement records. Most readers will be familiar with OK measurement procedures, but it may help to describe them briefly in order to indicate their limitations. A string, the 'base line', is stretched above the centre line of the inverted hull with the distance between the line and keel fixed at the transom, 200mm, and station 3, 28mm. All measurements which affect the hull's underwater shape are taken from this line using straight edge, spirit level, try square, plumb bob, and rule. To claim that with this measurement system all measurements can be guaranteed accurate to a millimetre, or even a fraction of a millimetre, is an exaggeration. Possibly two millimetres would be a more realistic margin. So we will not give actual measurement figures in this article as that would be too risky. We will record that a particular measurement is to maximum, to minimum or above/below the mean. Since most tolerances are set at plus or minus l0mm, that means we are working in a bracket of 2 or 3mm, probably accurate enough to give a clear idea of hull shape.
We have compared three basic hulls, all known to be competitive, the O'Donnell, the Skinner mould used variously by Skinner, Derbyshire & McMichael, Parker, Nelson & Omega, and the Hoare. We have only considered the measurements that affect the underwater shape.
The old, much quoted, rule of thumb in building the OK hull is to 'straighten the curves'. All three hulls do this, though to different extents. There has been much talk recently about 'fullness under the mast', but the validity of this concept is not universally accepted, and only the Hoare adopts this full bow policy fully. For a fuller discussion see Technical Manual 4, page 24.
First, the rocker. Remember the measurements at the transom and station 3 are fixed, leaving l0mm tolerances at stations 1, 2 and 3.5. All three hulls aim to maximise the waterline length by going for minimum measurement at station 3.5. Skinner achieves a flat rocker between stations 1 and 3 by going to minimum at station 1 and mean at station 2. This leaves a relatively steep slope from the transom to station 1. O'Donnell and Hoare have a fairly even fattish run from transom to station 3 by going for a maximum distance between base line and keel at both stations l. and 2.
The beam at the chine varies most between the three hulls at station 3, where Hoare aims to achieve the maximum fullness under the mast (located 500- 600mm forward of station 3) and O'Donnell and Skinner have very moderate ambitions in this direction. Working forward from the transom, the beam measurements run as follows. All three go to either maximum or above mean at the transom, Hoare goes to minimum at station 1 with the other two at or above mean, all three go very close to minimum at station 2, and while O'Donnell and Skinner revert to the mean or just above at station 3, Hoare goes for maximum beam at station 3.
As far as the height of the chine goes, the variation occurs once again largely at station 3, with Hoare going for minimum, the others for maximum. At the transom O'Donnell & Hoare go for minimum, Skinner for the mean. At station 1 Hoare and O'Donnell go for maximum, Skinner for the mean. At station 2 O'Donnell and Skinner go for the maximum, Hoare for the mean. At station 3, as mentioned above, O'Donnell & Skinner go for maximum, Hoare for minimum.
Now consider cross sections at the four measurement stations, generalising a bit. All go towards maximum beam and towards a relatively flat floor at the transom. All go towards minimum beam and maximum rise of floor at station 1. All go for minimum beam and maximum rise at station 2, though Hoare is less extreme than the other two on rise of floor. At station 3 Hoare goes for maximum beam and minimum rise, while the other two go for moderate beam and minimum rise. Skinner generally takes the most middle of the road approach, and in spite of this can claim a competitive record certainly equal to the other two. This is not to indicate any particular advantage over its rivals, just to point out that a 'moderate' design can be competitive.
Only the Hoare hull exploits the curvature rule to a marked degree, and that only at station 3. Some curvature is necessary when building in either wood or GRP to help provide stiffness in the bottom panels, but none of the builders we are considering seems to have attempted to use curvature for anything other than stiffness in the after part of the hull. One or two continental builders have but not, it seems, to any great effect in terms of boat speed. The O'Donnell and Skinner moulds use approximately 30-40% of the permitted curvature at all the transom and station 1, approximately 50% at stations 2 and 3. The Hoare mould was approximately 50% at the transom and station 2, approximately 75% at station 1, and is close to maximum at station 3. These curvatures need to be taken into account when viewing the comparative chine drawings that illustrate this article.
It is interesting to note how close to 'down the middle' the Skinner mould was designed. Of the measurements considered in this article only four stray far from the mean: minimum beam at station 2, maximum rise at stations 2 and 3, and minimum height from base line to keel at station 3.5.
Measurements other than those we have considered do not affect the under water shape and so probably have only marginal effect on performance. It may be an advantage to ensure maximum beam at the sheerline just aft of station 2, to maximise sitting out power, and to save weight and windage by keeping the sheerline low, but these are probably fairly minor advantages.
What we have tried to establish is that it is possible to build an OK of 'proven hull shape' without going to the extent of working to fractions of a millimetre, though of course many home builders would see that as being the challenge they wish to take on. If you build for yourself without these ambitions, you can still create for yourself a stiff, down to weight, seamanlike good looking vessel, capable of being competitive with any other OK on the water, and you can have a lot of fun and derive a lot of satisfaction from doing it.
This article was for the British reader, and so compared three British hull designs. The 'Skinner' hull is the one now produced by Rushworth Racing Dinghies.
It would be interesting to do a similar comparison with, say, a Hein, a Delf, a Botterill, a Cookson, and a Rushworth. Anybody offering?